Method and computer program for improving the dimensional acquisition of an object

ABSTRACT

The present invention relates to a method for improving the efficiency of dimensional acquisition of an object by a dimensional measurement device directed over the object, comprising the steps: a) directing the measurement device over the object to acquire its dimensions, b) providing an indication of the resolution of the acquired regions, c) re-directing the measurement device over at least part of the acquired regions indicating insufficient resolution according to predetermined criteria, d) updating the indication of the resolution of the acquired regions, and e) repeating steps c) and d) until sufficient resolution is indicated according to the predetermined criteria, thereby efficiently acquiring the dimensions of the object at sufficient resolution. It also relates to a computer program therefor.

This is a U.S. national phase of PCT Application No. PCT/EP2009/053931,filed Apr. 2, 2009, which claims the benefit of European Application No.08154793.7, filed Apr. 18, 2008 and U.S. Provisional Application No.61/046,153, filed Apr. 18, 2008.

BACKGROUND TO THE INVENTION

The invention relates to a method and computer program for thedimensional acquisition of an object required for reverse engineering orinspection tasks, which invention provides unskilled operators with anindication of acquisition quality the during the acquisition process.The inspection of physical objects usually comprises two differenttasks: the acquisition of a numerical representation of an object, andthe inspection of this numerical representation.

Dimensional acquisition (i.e. measurement) makes use of one or moredimensional measuring devices that are able to capture individual pointson the surface of the object. Such dimensional measuring devices arewell known in the art and might be, for example, a co-ordinate measuringmachine arm (CMM arm) equipped with a laser scanner. CMM arms aremanufactured by, for example, Faro Tech Inc, Hexagon Metrology AB orMetris. Laser scanners are currently manufactured by, for example, FaroTech Inc, Perceptron Inc, Kreon Technologies, and Metris. A CMM arm issometimes called a localizer because it is used to provide a 6dimensional location (i.e. position and orientation) of the laserscanner. Other kind of localizers also exist such as conventionalco-ordinate measuring machine (CMM) manufactured, for instance, by DEA,Mitutoyo, Wenzel, Zeiss, LK; or optical CMMs using light-emitting diodes(LEDS) or optical markers. Examples of optical CMMs include the MetrisK600, laser trackers, laser radars. Apparatus combining a localizer anda scanner also exists such as in the Handyscan product of Creaform Inc.

In a laser scanner, a laser plane intersects an object and theintersection is captured by an imager apparatus that employs a CCDcamera. Within the imager apparatus, the intersection is digitized in aseries of 2D points and given the position reported by the localizer,these 2D points are converted to 3D points representing points locatedon the surface of the object being acquired.

Other dimensional measurement devices can be used to acquire points onthe surface of an object, such a tactile probe (hard probes ofdeflecting probes) that acquires one point at a time; scanning probesthat acquire points continuously while the probe is moving over thesurface. Devices projecting structured lights also exist that are ableto acquire points on the surface of an object (e.g., GOM systems).

Computed tomography (CT) scanners or robot CMM arms are also measuringdevices able to measure points on the surface of the object. A CTscanner is also able to measure points on the internal surfaces of theobject.

The resulting acquisition comprises a point cloud (a set of 3D points)or any other alternative representation such as stereolithography (STL)meshes, surfaces or simply raw range images. In the case of CT scanners,the result of the acquisition may be 3D voxel data where each voxelscontains information about the density of the object.

These measuring devices may either be considered as manual devices i.e.a user is required to manipulate the device across the object to acquiredata; alternatively, they may be automatic i.e. the measuring device,based on a definition of a path, automatically passes over the object toacquire data without human intervention.

A computer aided design (CAD) model is a computer generated numericalrepresentation of an object to be manufactured. It results from buildingthe object within the confines of a software CAD application rather thanusing dimensional acquisition data directly. A CAD model can be arepresented by a set of points (a point cloud), a set of polygons (suchas an STL mesh) or one or more surfaces (Non-uniform rational B-spline(NURBS), B-Rep, analytical surfaces). The CAD model may also containadditional information that is critical for the manufacturing process ofthe actual object. Such information is usually manufacturing tolerancesand can be defined using the Geometric Dimensioning and Tolerancing(GD&T) ASME Y14.5M-1994 standard. The CAD model may also containinformation of features such as edges, circles, holes, slots, roundslots. These features may be represented by polylines, analyticalcurves, NURBS curves, etc . . .

During the inspection tasks, specific characteristics of an object canbe measured. For example, a user may want to verify the diameter of acircular hole or its position relative to another hole. He may also wantto verify the circularity of the hole. Another example of inspectiontask consists in the verification of the thickness between two surfaces.The user may also simply measure the distances between representativepoints of the surface. When a CAD model is available for the inspectiontasks, the point cloud may be compared entirely with the CAD model,features of the CAD model (nominal features) may be compared withfeatures of the point cloud (actual features).

The process of acquisition and inspection may be performed according totwo different workflows. In a first workflow, the acquisition andinspection tasks are decoupled. Here, a shop floor worker simplyacquires points on the surface of the object in order to capture thegeometry. When the acquisition is complete, the point cloud istransferred to the inspection specialist, usually a skilled worker, whowill perform the inspection tasks. This workflow suffers from somedrawbacks. Since the shop floor worker is not guided during theacquisition process, he may provide a point cloud that does not coverthe entire CAD model. Also, some regions may not be acquired withinsufficient accuracy or resolution. For example, by using a CMM armequipped with laser scanner, the shop floor worker is responsible tomove the scanner with the correct speed. If he is moving too fast, theresulting cloud will be undersampled and will miss small features (smallholes, fillets, edges, . . . ). Therefore, during the subsequentinspection, some inspection tasks will not be possible or will provideresults that are not reliable. The only remedy is to go back to the shopfloor and perform additional measurements which cost time or may even beimpossible if the object has already been displaced, shipped to anotherlocation. To avoid this problem, the shop floor worker usually tends toover-sample the object, which means that too many points are acquired.The resulting point clouds constitute very large files, containingredundant information. Both the acquisition tasks and the inspectiontasks require, therefore, extra time, and provide unwieldy files.

In a second (alternative) workflow, the acquisition and inspection tasksare performed sequentially, in that a part of the object is acquired,the acquisition stopped and an inspection is performed on that part. Theprocess of stop/start acquisition and inspection is repeated until theinspection of the object is complete. In an example of the procedure,the user is first required to measure points corresponding to a specificfeature (e.g. a slot) on the object, then to stop acquisition in orderto create this feature using the measured points. After acquiringseveral of these features, the user needs to stop acquisition again toproceed to an alignment task where the position of the objectsreferenced according to the reference system of the measuring machine isrelated to the reference system of the CAD model expressed in partcoordinate system. In the subsequent steps, the user is guided for theacquisition of the remaining points needed for the complete inspectionof the part, stopping between sessions of acquisition and inspection. Anexample of such workflow is described in e.g. US 2006/0274327 where theoperator is guided through a series of acquisition and inspection tasks.In US 2006/0274327, a new acquisition may be automatically requested tothe operator if the inspection has not succeeded.

The second workflow requires a skilled worker since he needs tounderstand the both the acquisition and inspection processes i.e. toacquire points with sufficient density and accuracy, and to then toperform feature detection, alignment, tolerances, etc as part of theinspection process. For example, some computer programs readily allowpoints to be continuously compared with a CAD model (assuming the workerhas already aligned the measuring system with the CAD model). However,these programs give a false sense of safety since they provideinformation on what has been acquired but not on what is missing or onthe quality of the acquired data.

In this workflow, the inspection software and acquisition software areidentical to that of the first workflow, thus creating complexity sincethe user needs to master two complex software applications. Further,time is wasted because the user has to stop acquisition and switch backand forth between acquisition and inspection software application andcontinuously transfer the newly acquired 3D points. Finally, the usermust be able to read and understand the CAD data, correctly identify onthe object the features or the location where points need to beacquired.

The acquisition of points on the surface of 3D objects may also be usedin the context of reverse engineering. Here, the acquisition is guidedby the need to acquire geometric information on the surface of theobject with sufficient density as to be able to create a globalnumerical model, e.g. by means of NURBS surfaces or STL mesh. Too fewacquired points will create holes in the model which will result in amodel of poor quality where details are missing. Too many points willslow down the reverse engineering tasks since most of the reverseengineering operations require a time proportional to the number ofpoints. Such a model can be later used for rapid prototyping, orvisualization.

SUMMARY OF THE INVENTION

The present invention concerns a new method and computer program forimproving the accuracy and speed of dimensional acquisition of a 3Dnumerical representation of an object; the invention allows subsequenttasks such as inspection or reverse engineering to become possible andefficient.

The present invention provides a method for improving the efficiency ofdimensional acquisition of an object by a dimensional measurement devicedirected over the object, comprising the steps:

-   a) directing the measurement device over the object to acquire its    dimensions,-   b) providing an indication of the resolution of the acquired    regions,-   c) re-directing the measurement device over at least part of the    acquired regions indicating insufficient resolution according to    predetermined criteria,-   d) updating the indication of the resolution of the acquired    regions, and-   e) repeating steps c) and d) until sufficient resolution is    indicated according to the predetermined criteria,    thereby efficiently acquiring the dimensions of the object at    sufficient resolution.

Alternatively expressed, the invention provides a method for improvingthe efficiency of the dimensional acquisition of an object by adimensional measurement device directed over the object, comprising:

-   -   directing the measurement device over regions of the object to        acquire their dimensions,    -   providing, during acquisition, a continuous indication of the        resolution of the acquired regions, and    -   directing the measurement device, during acquisition, over        regions indicating insufficient resolution according to        predetermined criteria,        thereby efficiently acquiring the dimensions of all regions of        the object at sufficient resolution.

The method thus provides feedback during acquisition. The feedback i.e.updating in step d) may be provided continuously, meaning at regular orirregular time intervals, for example at a certain frequency such as 1Hz, 10 Hz, 10 kHz, or on user request (e.g. by pressing a button). Aregion has an ordinary meaning, herein, referring to a part of theobject being acquired. For example, a region may be defined as one ormore points, curves or surfaces. The regions need not necessarily coverthe entire object. For example, if an inspection task is restricted tothe determination of a single circular feature, one single regiondefined around this feature is required. The indication may show whetherthe resolution is sufficient or insufficient according to predeterminedcriteria. It is noted that resolution preferably refers to an acquiredregion, and not to non-acquired (not yet scanned) regions of the object.A sufficient resolution of a region means that the acquired dataprovides enough information to be able to perform the subsequent taskse.g. detect a circular hole and verify if the diameter of the hole iswithin prescribed tolerance compare to the nominal diameter. Aninsufficient resolution, therefore, implies the opposite. For example,if the noise is zero, sufficient resolution for the inspection of acircular hole can be expressed as at least 3 points, since 3 points areneeded to describe a circle. If the noise is 25 mu and the tolerance 50mu, sufficient resolution could mean at least 20 points since using 20points there is more than 95% chance of answering correctly to thequestion whether the diameter is within tolerance. The sufficiency orinsufficiency is thus determined by criteria that may vary according tothe shape of the regions being acquired, according to the measurementdevice, subsequent tasks to be performed (such as inspection or reverseengineering) and other factors. The criteria may be predetermineddynamically during acquisition, or set by the user prior to acquisition.The indication of resolution provides a sign to the user or automatedmeasurement system to direct the measurement device, during acquisition,over regions indicating insufficient resolution according to thesepredetermined criteria.

The invention also provides a computer program stored on a computerreadable medium capable of performing the method as described above. Theinvention provides a computer program for improving the dimensionalacquisition of an object by a dimensional measurement device directedover the object, that comprises a “coverage module” (or coveragesoftware routine) and “feedback module” (or feedback software routine)stored on a computer readable storage medium—explained in detail below.The program runs on a processing means such as a computer during theacquisition process. The coverage module receives the acquired data ofthe measuring device at regular intervals and estimates which regions ofa CAD model are sufficiently covered by the acquired data and/or whichregions of the CAD model have insufficient coverage. Coupled to thecoverage module, the feedback module provides feedback to the user or tothe measuring device concerning the regions of the object where furtherdata acquisition is needed and where data acquisition is sufficient. Forexample, if an operator uses a manual CMM arm with a laser scanner, thefeedback module may present the CAD model on a computer screen where theregions with missing data are coloured red. Alternatively, when themeasuring device is automatic (e.g. a conventional CMM with a laserscanner, or a robot CMM arm with a laser scanner), the feedback modulemay provide feedback in the form of a list of CMM/laser scannerlocations required to acquire data in regions with insufficientresolution.

Using the invention with a manually-operated measuring device, a typicalworkflow entails that a CAD model is first loaded in the acquisitionsoftware, and the coverage module and feedback modules are started. Ifnecessary, an optional alignment between the object in measuring devicereference system and CAD reference system may be performed.

Prior to acquisition, a CAD model, or part of a CAD model, may bepresented on the screen of the computer that is indicated with markings,for example, with red shading, meaning that no data has been acquired.The user is simply requested to acquire data on the object in any order.He may choose to proceed as a painter would proceed while painting theobject. Regularly, the CAD model is re-rendered on the screen whereregions having been acquired to sufficient resolution are indicated inan alternative way, for example being shaded green. At anytime duringthe acquisition, the user may look at the screen to identify the redregions and further acquire data on the corresponding location of theobject. At the end of the process, the entire CAD model is colouredgreen and the acquisition may stop.

Alternatively, prior to acquisition, a CAD model may be presented on thescreen of the computer that is indicated with markings, for example,with red shading, meaning that no data has been acquired. The user isrequested to acquire data on the object in any order. He may choose toproceed as a painter would proceed while painting the object. Regularly,the CAD model is re-rendered on the screen where regions having beenacquired to sufficient resolution are indicated in an alternative way,for example being shaded green. Those regions that have been acquired,but at insufficient resolution may be shaded in an alternative way, forexample, amber, indicating of the resolution of the model in that regionis insufficient and that the measurement device should be directedthereover. Those regions still require a first pass by the measurementdevice may be shaded red. At anytime during the acquisition, the usermay look at the screen to identify the red regions which still require afirst pass by the measurement device over the corresponding location ofthe object, amber regions which require further passes over thecorresponding location of the object, and green regions that do notrequire any further passes by the measurement device. At the end of theprocess, the entire CAD model is coloured green and the acquisition maystop.

Based on the CAD model and the acquired data, the coverage module of thesystem computes whether a particular region or detail of the CAD modelhas been acquired with sufficient and/or insufficient resolution. Asufficient or insufficient resolution is determined by the shape of theobject, the type of measurement device and the subsequent tasks to beperformed on the object as mentioned elsewhere. Typically it isdetermined according to the number of points to be acquired on aparticular region. For example, a density of one point every 1 mm² maybe required by the user, and this represents part of the criteria.Alternatively, a varying point density may be chosen when more pointsare required in regions with higher curvature. Alternatively, a higherdensity of points may be asked along the edges of the features.Alternatively, the number of points may be dependent on a user giventolerance or in a tolerance present in the CAD file (CATIA) or accordingto GD&T requirements. Alternatively, the number of points may be adapteddynamically according to the type of probe. The use of an accurate touchprobe will provide an accurate coverage, faster than a laser probe.

In the invention, the measuring device is preferably manual measuringdevice, either a CMM arm and a laser scanner, or a laser scannercombined with an optical CMM. In this case, the feedback module rendersinformation on the computer screen. The user may also choose anautomatic measuring device. In this latter case, the feedback model maydirectly drive the automatic measuring device and make it acquire datain the regions with insufficient coverage.

Some particular embodiments of the invention are described below:

One embodiment of the invention is a method as described above, whereinsaid indications are provided on a computer aided design, CAD model ofthe object.

Another embodiment of the invention is a method as described above,wherein the CAD model and indication of resolution for acquired regionsof the object are displayed using a display means.

Another embodiment of the invention is a method as described above,wherein the indication of resolution comprises an indication of acquiredregions having a sufficient and/or insufficient resolution according tosaid predetermined criteria.

Another embodiment of the invention is a method as described above,wherein acquired regions of sufficient resolution are indicated on theCAD model using a first marking, and acquired regions of insufficientresolution are marked on the CAD model using a second marking that isdifferent from the first marking.

Another embodiment of the invention is a method as described above,wherein the indication of the resolution comprises an indication ofacquired regions having the same resolutions.

Another embodiment of the invention is a method as described above,wherein the CAD model and indication of resolution for the acquiredregions of the object are displayed using a display means and whereinregions having the same acquired resolution are indicated on the CADmodel using a common marking, and where regions of differently acquiredresolutions are marked with different markings.

Another embodiment of the invention is a method as described above,wherein the indication of resolution is further updated after themeasurement device acquires a region that at least partially overlaps apreviously acquired region.

Another embodiment of the invention is a method as described above,wherein the measurement device is a manually operated measurementdevice.

Another embodiment of the invention is a method as described above,wherein the measurement device comprises an optical scanner and either aco-ordinate measuring machine, CMM, arm or an optical CMM.

Another embodiment of the invention is a computer program stored on acomputer readable medium, configured to perform a method describedabove.

Another embodiment of the invention is a computer program stored on acomputer readable medium, for improving the dimensional acquisition ofan object by a dimensional measurement device that is directed over theobject, configured to:

-   a) receive three-dimensional data of the object from the measurement    device directed over regions of the object,-   b) provide an indication of the resolution of the acquired regions,-   c) receive additional three-dimensional data of the object from the    measurement device for at least part of the acquired regions    indicating insufficient resolution according to predetermined    criteria,-   d) update the indication of the resolution of the acquired regions,    and-   e) repeat steps c) and d) until sufficient resolution is indicated    according to the predetermined criteria.

Another embodiment of the invention is a computer program as describedabove, further comprising a coverage module configured to receive thethree-dimensional data of the object from the measurement device duringdimensional acquisition, and to provide the updated indication of theresolution of the acquired regions.

Another embodiment of the invention is a computer program stored on acomputer readable medium, for improving the dimensional acquisition ofan object by a dimensional measurement device that is directed over theobject, comprising a coverage module configured to receivethree-dimensional data of the object from the measurement device duringdimensional acquisition, and to provide a continuously updatedindication of the resolution of the acquired regions.

Another embodiment of the invention is a computer program as describedabove further comprising a feedback module configured to provide saidindication of resolution of the acquired regions on a CAD model or asdata to an automated measurement device.

Another embodiment of the invention is a computer program as describedabove, wherein the indication of resolution of the acquired regionscomprises an indication of acquired regions having sufficient and/orinsufficient resolution according to predetermined criteria.

Another embodiment of the invention is a computer program as describedabove, comprising a feedback module configured to provide saidindication of resolution of the acquired regions on a CAD model, whereinacquired regions of sufficient resolution are indicated on the CAD modelusing a first marking, and acquired regions of insufficient resolutionare marked on the CAD model using a second marking that is differentfrom the first marking.

Another embodiment of the invention is a computer program as describedabove, wherein the indication of the acquisition resolution comprises anindication of acquired regions having the same resolutions.

Another embodiment of the invention is a computer program as describedabove, wherein acquired regions having the same resolution are indicatedon the CAD model using a common marking, and where acquired regions ofdifferent resolutions are marked with different markings.

Another embodiment of the invention is a computer program as describedabove, further comprising a display means to display the resulting CADmodel.

Another embodiment of the invention is a computer program as describedabove, further comprising a sub-routine to indicate when the dimensionalacquisition of the object to a sufficient resolution according topredetermined criteria is complete for all acquired regions of theobject.

FIGURE LEGENDS

FIG. 1 Schematic illustration of an embodiment of the method of theinvention.

FIG. 2 Schematic illustration of the updates to the CAD model duringacquisition of the object according to a method of the invention.

FIG. 3 Flow chart indication of an embodiment of the method of theinvention.

FIG. 4 Flow chart indication of an embodiment of the coverage module ofthe invention.

FIG. 5 Flow chart indication of an embodiment of the feedback module ofthe invention.

FIG. 6 Flow chart indication of the transfer of data from the renderingengine of FIG. 5 to a screen or a measurement device.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as is commonly understood by one of skill in theart. The articles “a” and “an” are used herein to refer to one or tomore than one, i.e. to at least one of the grammatical object of thearticle. The recitation of numerical ranges by endpoints includes allinteger numbers and, where appropriate, fractions subsumed within thatrange (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, forexample, a number of articles, and can also include 1.5, 2, 2.75 and3.80, when referring to, for example, measurements). The recitation ofend points also includes the end point values themselves (e.g. from 1.0to 5.0 includes both 1.0 and 5.0)

The present invention provides a method for improving the efficiency ofdimensional acquisition of an object by a dimensional measurement devicedirected over the object, comprising the steps:

-   a) directing the measurement device over regions of the object to    acquire their dimensions,-   b) providing an indication of the resolution of the acquired    regions,-   c) re-directing the measurement device over at least part of the    acquired regions indicating insufficient resolution according to    predetermined criteria,-   d) updating the indication of the resolution of the acquired    regions, and-   e) repeating steps c) and d) until sufficient resolution is    indicated according to the predetermined criteria,    thereby efficiently acquiring the dimensions of the object at    sufficient resolution.

The updating of step d) takes account of the newly acquired data in stepc). By repeating steps c) and d), the resolution of acquired regions isincreased with each pass of the measurement device until it issufficient according to the predetermined criteria. It will beappreciated that d) need not be performed directly after step c) i.e.intervening steps may be employed in which the measurement device isdirected over regions other than those having been acquired andindicating insufficient resolution, for instance, regions not yetacquired.

In an alternatively expressed embodiment, the invention provides amethod for improving the efficiency of the dimensional acquisition of anobject by a dimensional measurement device directed over the object,comprising:

-   -   directing the measurement device over regions of the object to        acquire dimensions of the regions,    -   providing, during acquisition, a continuous indication of the        resolution of the acquired regions, and    -   directing the measurement device, during acquisition, over        regions indicating insufficient resolution according to        predetermined criteria,        thereby efficiently acquiring the dimensions of all regions of        the object at sufficient resolution.

The method thus provides feedback during acquisition of data, whichfeedback is a time-continuous indication of the acquisition resolutionof the regions, more preferably of regions having sufficient orinsufficient resolution according to predetermined criteria. It providesan indication to the user or automated measurement system to direct themeasurement device, during data acquisition, over regions indicatinginsufficient resolution according to these predetermined criteria. Thesteps above are useful for acquiring the dimensions of an object at aresolution suitable for a subsequent inspection; it avoids discovery ofinsufficient resolution at the costly inspection stage.

The invention also relates to a computer program for improving thedimensional acquisition of an object by a dimensional measurement devicethat is directed over the object, comprising a “coverage module”configured to receive at least three-dimensional data of the objectduring dimensional acquisition, and to provide, during dimensionalacquisition, a continuous indication of acquisition resolution of theobject, whereby regions having the sufficient resolutions are indicatedfor the object.

The computer program may further comprise a “feedback module” configuredto receive the indication of the acquisition resolution for regions ofthe object and to provide said indication on a CAD model.

As mentioned elsewhere, the task of dimensional acquisition is known inthe art, and makes use of one or more dimensional measuring devices thatare able to capture individual points on the surface of the object. Theygenerally comprise a localizer that accurately reports the position of ameasurement probe. Dimensional measuring devices are well known in theart and any are within the scope of the invention. Examples include aCMM arm (manufactured by, for example, Faro Tech Inc, Hexagon Metrologyor Metris) equipped with a laser scanner (manufactured by, for example,Faro, Perceptron, Kreon, and Metris). Other examples include aconventional CMM (manufactured, for instance, by DEA, Mitutoyo, Wenzel,Zeiss, LK) or an optical CMM equipped with a measurement probe.

In a laser scanner, elaborated elsewhere, a laser plane is projectedonto an object and the projection is captured by an imager apparatusthat employs a charged-couple device (CCD) camera. Within the imagerapparatus, the projection is digitized into a series of 2D points andgiven the position reported by the localizer, these 2D points areconverted to 3D points representing points located on the surface of theobject being acquired.

Other dimensional measurement devices can be used according to theinvention to acquire points on the surface of an object, such thosecomprising a tactile probe (hard probes or deflecting probes) thatacquires one point at a time; such those comprising a scanning probethat acquire points continuously while the probe is moving over thesurface. Devices projecting structured lights also exist that are ableto acquire points on the surface of an object (e.g. manufactured by GOMsystems).

Other dimensional measurement devices according to the invention includecomputed tomography (CT) scanners or robot CMM arms. A CT scanner isalso able to measure points on the internal surfaces of the object.

Other dimensional measurement devices can be used according to theinvention to acquire dimensional data in a form other than points on theobject surface. U.S. Pat. No. 6,616,617 discloses a method where a laserscanner provides directly triangle strips. Computer tomography scannersmay provide voxel (volumetric) information.

These dimensional measurement devices may be equipped with additionalprobes to obtain additional information during the acquisition. Examplesof such probes include colour cameras, temperature sensors or ultrasoundsensors for acquiring wall thickness information. It will be understood,therefore, that acquisition may not be limited to surface dimensionalmeasurement, but may include also capture of color, texture and depth(thickness) information.

These measuring devices may be manually operated devices i.e. a user isrequired to manipulate the device over the object to acquire data;alternatively, they may be automatic i.e. the object and measurementdevice move automatically relative to each other. In the latter case,the measuring device, based on a definition of a path, may automaticallypass over the object to acquire data without human intervention.Alternative, the measurement device may be static while the objectitself is in movement, for example by use of a rotary table.

The resulting acquisition comprises a point cloud (a set of 3D points)or any other alternative representation such as STL meshes, surfaces orsimply raw range images. In the case of CT scanners, the result of theacquisition may be 3D voxel data where each voxels contains informationabout the density of the object. When additional probes are used, theresulting acquisition also includes the additional information providedby the additional probes.

Reference is made in the description below to the drawings whichexemplify particular embodiments of the invention; they are not at allintended to be limiting. The skilled person may adapt the method andprogram and substitute components and features according to the commonpractices of the person skilled in the art.

The invention is described in greater detail with reference to FIG. 1.The object 10 that needs to be acquired is placed in the measuringvolume of the measuring device which comprises a laser scanning probe 9attached to a CMM arm 8 fixed to a base 7. A computer program, stored ona computer readable medium, instructs a processing device 1 tocommunicate with the CMM arm 8 and the laser scanner 9 typically via acommunication cable 6. The program is able to receive data from the CMMarm 8 and the laser scanner 9 and convert the data to 3D datarepresenting points on the external (or internal surfaces) of theobject. A CAD model 11 is loaded into the program and displayed on thecomputer display 2. The operator 14 manually passes the probe 9 over theobject 10; a laser line 12 is projected on the object 10, and itscorresponding numerical representation 13 is displayed on the CAD model11 which line is detected and establishes the measurements of theobject. Prior to acquisition, the CAD model 11 has entirely (not shown)a dark shading indicating the regions to be acquired by passing theprobe. As the acquisition progresses, the dark shading 21, 21′ isreplaced by regions of lighter shading 20, indicating that theresolution of the acquisition has met or exceeded predetermined criteriain these lighter 20 regions. By observing the computer display 2 duringacquisition, the operator 14 recognizes the regions 21, 21′ that need tobe passed or passed again with the probe 9.

An example of how the model is updated during measurement is given inFIGS. 2A to F. A CAD model 11 is loaded into the program and displayedon the computer display 2. In FIG. 2A, the CAD model has entirely darkshading 21 indicating the regions not yet acquired by passing the probe.The operator manually passes the probe in the direction of the arrowover the object. The CAD model updates in FIG. 2B; the dark shading 21is replaced by regions of cross-shading 20, indicating that theresolution of the acquisition has met or exceeded predeterminedcriteria. It is also replaced by a region of hatched shading 23,indicating said region has been acquired, but the resolution of theacquisition has not met predetermined criteria. The operator manuallyredirected the probe over the acquired region 23 indicating insufficientresolution in the direction of the arrow in FIG. 2C. The CAD modelupdates in FIG. 2D; the hatched shading is replaced by a region ofcross-shading 20, indicating that the resolution of the acquisition hasnow met or exceeded predetermined criteria. The operator manuallyredirected the probe in the direction of the arrow over a previouslyunacquired region 21 in FIG. 2E. The CAD model updates in FIG. 2F; thedark shading is replaced by a region of cross-shading 20, indicatingthat the resolution of the acquisition has met or exceeded predeterminedcriteria. Thus, by observing the updating display 2 during acquisition,the operator recognizes the regions that need to be passed or passedagain with the probe 9.

As mentioned elsewhere, prior to the acquisition, the user may firstload a CAD model 11 representing the object into the computer program.Such a CAD model may be any kind, for example, it may be defined as anSTL model, a NURBS surface using a B-rep representation, analyticalsurfaces or any other format suitable to represent surfaces, curves orpoints numerically. For example, the CAD model may have been generatedusing the CATIA system developed by Dassault Systèmes. The CAD model mayalso be a point cloud or a mesh of a previously acquired object. Inaddition to the mere representation of the surfaces of the object, theCAD model may also contain additional information on how the objectneeds to be inspected. An example of such additional information is thetolerances on the features such as defined by GD&T standard defined inthe ASME Y14.5M-1994.

With reference to FIG. 3, after the user has loaded the CAD model 110 hemay optionally align 130 the physical object with the CAD model prior toacquisition. To do this, he first measures features 120 of the object,for example, by first measuring and detecting three circles of theobject, selecting the corresponding circles on the CAD model and usingstandard alignment techniques, compute a transformation matrix 121 thatpositions the actual circles onto the corresponding nominal circles.This may be performed by the computer program of the invention. Numerousother techniques also exist to produce an initial alignment. Forexample, other type of features, such as planes may be used. Thetransformation matrix may be read from a file (if more than one similarobjects needs to be scanned, an alignment is only performed once andsaved to disk). The transformation may be manually entered via akeyboard. The user may also choose to manually align a portion of theactual model with the CAD model on the screen using his mouse while thecomputer software incrementally updates the resulting transformation.

When the optional alignment 130 is complete, the coverage 140 module ofthe present invention provides a continuous and updated coverageinformation 200 which information comprises an indication of the regionsof the object that have been acquired with sufficient and/orinsufficient resolutions. Regions of sufficient or insufficientresolution may be calculated according to predetermined criteria asmentioned earlier. The coverage information 200 is sent to the feedback150 module for presentation of a CAD model. Regions of sufficientresolution maybe indicated on the CAD model using a first marking (e.g.green), and regions of insufficient resolution may be marked on the CADmodel using a second marking (e.g. red) that is different from the firstmarking.

With reference to FIG. 4, the coverage module 140 receives a descriptionof the CAD model, optional user parameters and optional informationabout the measuring device. During a typical process, the coveragemodule 140 receives data information 142 (e.g.

points, colour, . . . ) from the measurement device at regularintervals. In response to this, the module 140 updates 143 the coverageinformation, which information 200 can be sent 144 to the feedbackmodule 150 where it can be presented on the CAD model. Using the updatedcoverage information, the coverage module 140 is able to identify 145whether the current set of acquired points is sufficient to describeaccurately a portion of the CAD model. In addition, the coverage module140 is able to provide information on the portion of the CAD model thatis sufficiently acquired or on the portion of the CAD model that is notsufficiently acquired according to predetermined criteria. If coverageis complete (Y) then a termination signal 201 can be sent 146, otherwise(N) the module 140 continues to process information received from themeasurement device.

If the CAD model is an STL mesh, the coverage information 200 maycomprise a values ranging from 0 to 100% for each triangle of the meshwhich values correspond to the resolution of the triangle acquired. Avalue of 0 may mean that the corresponding triangle on the object hasnot been sampled yet while a value of 100% may mean that thecorresponding triangle has been fully sampled. On request from thefeedback module 150 or at its own frequency, the coverage module 140 maysend 144 the coverage information 200 to the feedback module 150. Thisinformation may concern the entire CAD model, or the informationmodified since the previously sent information.

The feedback module 150 runs concurrently with the coverage moduleeither on the same computer or on another computer. It may be linked toa computer screen as shown in FIG. 1 and be able to render informationon the screen. With reference to FIGS. 5 and 6, after reception ofcoverage information 200 from the coverage module 140 and using thegeometrical information of the CAD model, the feedback module 150computes 151 rendering information that is sent to the rendering engine152. The render engine 152 transforms the information into, for example,into a rendered CAD model for transfer 300 to a screen 302 for displayas shown in FIG. 6, additionally or alternatively into machine locationinformation for transfer 300 to the automatic measurement device whereemployed such as a controller for a CMM arm 304, or to any otherutilized automatic measurement device 306.

Where the rendering engine 152 render CAD model for display on a screen203, the model may be rendered with various indications, preferablycoloured shading, mesh or points to indicate coverage information. Forexample, regions that are sufficiently covered may be coloured green andthe remainder of the CAD model, not sufficiently covered, may remainred. It is within the scope of the invention that more than two colorsare used denoting partial coverage e.g. green for portions sufficientlycovered, red for portions not yet covered, amber for portions coveredbut not at sufficient resolution. Alternatively, a continuous colour mapmay be used indicating coverage resolution. In a further alternative,the feedback module 150 may render only the portions that are notcovered. The portions sufficiently covered are not rendered or may berendered with some transparency factor. Instead of using colors, thefeedback module may use different textures to represent the coveringinformation. If the feedback module 150 receives a signal 201 thatcoverage is complete (Y) then it may indicate completion by way of thedisplay 2, otherwise (N) the module 150 continues to render the CADmodel.

When using an automatic measuring device, the feedback module 150 is notnecessarily connected to a display device but may additionally oralternatively be connected to some other devices linked to the measuringdevice. For example, it may be linked to the controller of the measuringdevice 304, 306. In this case, the rendering computation 151 computesthe region that needs to be further acquired and the rendering engine152 converts this region to a measuring device location that is sent 300to the controller. The controller then moves the measuring device 304,306 to the given location and proceeds to an additional acquisition. Inan alternative workflow, the rendering engine 152 simply provides therendering information or part of the rendering information that anexternal device or program whose responsibility is to control themeasuring device. These programs, also called “path programming”programs are known in the art.

The feedback module 150 may run synchronously with the coverage module140 and render the rendering information every time coverage data isreceived. Alternatively, the feedback module 150 may be asynchronous.For example, the rendering 152 may be performed at a given frequency.Also, the rendering engine 152 may be utilised only when the renderinginformation 151 sufficiently differs from the previous renderinginformation. In still another alternative, the rendering engine 152 maybe utilised when the operator provides an instruction thereto, forexample, by pressing a button (located on the keyboard, or on themeasuring device).

During the acquisition, the user typically paints on the surface of theobject as if he would be painting the object. The speed at which theinformation is acquired, and the order of the information is notrelevant to the invention; the required resolution is achievedregardless. The acquired data is then transferred to the coverage module140 that will in turn update the coverage information and send theupdate information to the feedback module 150. Regularly, during theacquisition process, the user may decide to check on the screen whichportions of the object remain to be acquired. In this process, he may beguided by interacting with the feedback module. The feedback module mayzoom or rotate toward the portions where additional data needs to beacquired.

The covering information may be computed by the coverage module 140using various methods. For example, if the CAD model is an STL modelwith triangles, each triangle is assigned an integer value P. Eachacquired point is first projected onto the closest triangle and thevalue P of the triangle is incremented. It may be decided that thetriangle is fully covered when P exceeds a certain threshold (say 10points). This threshold may be hard-coded, given by the user or computedby the coverage module. It may be fixed for all triangles or depend onthe area of the triangle or depend on the local curvature of thetriangle. The value P may be incremented by other values than 1. If 3points located on a line are projected onto the same triangle, thecoverage module may choose not to increment at all when receiving the3rd point since it does not provide additional geometrical information.If the user requested a point density of 1 point every millimeter, thecovering information may have the form of a regular grid of 1 mm²squares distributed on the surface of the CAD model where each square isassigned a value of either 0 (not covered) or 1 (covered).

If additional probes are present, the coverage module 140 may use thedata acquired by the additional probes to update the coveringinformation. For example, the availability of colour information in atriangle may be used to modify the integer value P of this triangle.

If additional information such as GD&T information is also present inthe CAD model, it can be used to compute the covering information. Forexample, if during inspection, the position of a planar surface must becalculated, a few points are required on this surface to detect theplane. If the planarity of the surface is also necessary, more pointsmust be acquired. The number of points may also depend on the tolerancegiven in the GD&T information. The coverage module 140 may compute thenumber of points a priori or adapt the number of points duringacquisition based on the already acquired points on the plane.

Other types of methods can be used to define the covering information:more than one threshold may be used; a continuous function may bedefined on the triangle hence defining different covering information ineach point of the triangle. Also, when a point is projected on atriangle, it may also modify the coverage information of neighboringtriangles. Alternatively, a point may be projected on more than onetriangle.

When other types of CAD model are used, covering information may beassigned to individual points, to edges, to features or to surfaces. Thecovering information may be defined on 2D surfaces, 1D edges orindividual points.

When receiving data from the measuring device, the coverage module 140calculates whether the coverage is complete 145. For example, when 99%of the triangles have an index P exceeding the threshold. At thismoment, the coverage module may send a special signal 201 that coverageis complete. The feedback module may react to this signal, for exampleby showing a dialog box on the screen notifying the completion of theprocess. Other types of feedback may be used, not necessarily using thefeedback module. For example an audible signal may be given or a greenlamp attached to the ceiling may flash. It should be noted that thecompletion of the acquisition may trigger other type of operations. Forexample, the numerical representation of the object may be saved to diskor emailed to the person responsible for the inspection. Alternatively,if an inspection software is installed on the computer and is able torun automatically, it may be started. In addition, if the object isplaced on a conveyer belt, the conveyer belt may be requested to placethe following object in the measuring volume of the measuring device.

The above preferred embodiments has been described using a measuringdevice comprises of a CMM arm and a laser scanner. Other type ofdimensional measuring devices may be used such as the one that useoptical localizers and laser scanners (Metris K-Scan, CreaformHandyScan). The user may also use other types of scanners, touch probesor scanning probes as discussed above. A user may use the joystick of aconventional CMM to move a scanner around an object to be acquired. Hemay also use a Robot CMM arm or any other measuring machines that may beused in a manual way.

The coverage module may be used to give qualitative (e.g. two-tone) orquantitative (e.g. colour scale) coloring to the CAD model.Alternatively, the module may give a geometric definition of theportions of the CAD that remain to be scanned. Therefore, the inventionmay also be used with automatic measuring devices. During manualacquisition, the feedback module is linked to the computer screen. Itcan be also linked to the controllers of the automatic measuring devicesas mentioned earlier. Based on the coverage information provided by thecoverage module, the feedback module may compute the path of theautomatic measuring device that can provide data in regions where thecoverage is not sufficient. The path information is then sent to thecontroller. In this automatic mode, only the definition of the measuringdevice is needed. There already exist software programs that createcomplex paths for the automatic acquisition of complex objects. However,these software programs create the paths a priori. There is no feedbackto update the path once the acquisition has started. These softwareprograms are not fault tolerant and may then fail if, for example, theobject is not correctly aligned. In this invention, the path may bedefined incrementally by the feedback module since there is a feedbackloop.

The display means may be a screen 2, 302 (e.g. LCD, OLED, CRT, plasma)connected to the computer as shown in FIGS. 1 and 6. The location of thedisplay means is preferably in the line of sight of the operator, forexample, adjacent to the object being acquired or located in the rear ofthe scanner unit for viewing by the operator simultaneous with scanning.Other display means may be employed. For example, it may be an imageprojector, configured to project an image directly on the object beingscanned, which image projector may be in a fixed location, for example,on the ceiling of the shop floor or it may be located in the projectingend of the moving probe. Such projectors may be color DSP projectors orlaser projectors, such as manufactured by Virtek. Image projection iswell known, including how to align an object with the projector (usinge.g. an iGPS system) and how to correct the image when the object is notplanar. The image may contain color information (e.g. red and green) butalso text. When an image projector is used, the user no longer needs tolook at a screen during acquisition. He simply needs to “paint” theobject as long as some color (for example red) is projected onto it.

In order to further automate the process, the object may be equipped byan RFID label and the computer or the measuring device may be able toread this RFID and in response load the corresponding CAD model.

The preliminary step of aligning the CAD model with the object is notmandatory. The alignment may be computed dynamically as the points arebeing acquired. During the initial part of acquisition, the number ofpoints may be too low as to provide a good alignment transformation withsufficient certainty. The coverage module hence does not provide anycovering information. As more and more are acquired, improved alignmenttransformations are computed, for example using the well known iterativeclosest point algorithm (ICP). Once the alignment is sufficient, thecoverage module may start providing covering information. It should benoted that contrary to the other existing alignment methods, theautomatic alignment need not be very accurate. Indeed, during theinspection task, the accuracy of the alignment influences directly theresult of the inspection. In our case, the alignment is only used toobtain and rough correspondence between acquired points and CAD model. Amisalignment of a few millimeters is still acceptable.

The coverage module 140 may accept additional information from themeasuring device. Such information could be an estimation of theaccuracy of the points. Based on this information, it could update thecovering information. For example, if acquired points are labeled ashaving an accuracy of 1 mu, the coverage module may compute that 4points are sufficient to detect a circular feature. On the other hand,if the points have an accuracy of 25 mu, the module may require 20points or more and update the coverage information accordingly

It should be noted that for reverse engineering activities, a CAD modelis not available. Nonetheless, the method can still be used. Indeed,during acquisition, the data obtained can be meshed in real time (asdisclosed in U.S. Pat. No. 6,616,617) and the mesh can then be used toprovide an approximation of the CAD model. If the user requested a givenpoint density, the triangles may be directly coloured depending on theirsizes. If a curvature dependant density is preferred, the coveragemodule may first compute an approximate value of the curvature dependingon the succession of stripes.

For a reverse engineering task for the film industry, for example, thecolor may be important. Therefore, the coverage module may also checkthat the color sampling of the object is sufficient, using data obtainedfrom a color camera simultaneous with data obtained from the measurementprobe. Alternatively, the corrosion analysis of a pipeline requiresinformation of the wall thickness at sufficient intervals. The coveragemodule may thus take the thickness information into account whencomputing the coverage information.

The above description has been made by assuming that the acquisitiondata of the measuring devices are points. Of course, the above methodcan be extended to other types of data such a polygons, range images,voxels from CT scans. The coverage module computes whether the data issufficient to perform the various inspection or reverse engineeringtasks with sufficient accuracy. It is not needed to perform the actualinspection or reverse engineering. For example, using CT data, thecoverage module verifies the voxel size and density distribution on thevoxels. Based on this information, the coverage module may estimate thatthe subsequent surface reconstruction will be sufficiently accurate toallow the detection of features. If only features are needed during theinspection task, the coverage module then notifies that the covering iscomplete.

The coverage module 140 may be incorporated into a microprocessing unitlocated in the base of the measuring device. The communication to themeasurement device and the feedback module may be wireless. Thecomputation of the 3D position of the points may be located in the base7 and not on the computer 1.

Prior to the acquisition, regions of the CAD model may already show thatthe resolution is sufficient, for example if no data is required inthese regions for the inspection process or if data has already beenpreviously acquired (maybe with another measuring device)

The above method can also be applied with 2D measuring devices (such aprofilometers) and 2D CAD models or 2D sections of 3D models.

For every point, the coverage module 140 updates the coveringinformation. If the covering information does not change or is onlyupdated slightly, this means that the particular does not addinformation. This may happen if the point is a duplicate of an alreadyacquired point, or is the region was already sufficiently acquired. Whenthis happens, the point need not be kept in the point cloud and may bereadily discarded. Therefore, the total size of the pointcloud may becontrolled even in the event that the operator passes several times overthe same region of the object.

The invention claimed is:
 1. A method for the dimensional acquisition ofan object at a resolution sufficient for a subsequent inspection taskusing a dimensional measurement device manually directed over the objectby an operator, comprising the steps: a) manually directing themeasurement device over regions of the object to acquire theirdimensions, b) providing during acquisition, on a nominal computer aideddesign (CAD) model of the object, an indication to the operator ofwhether the resolution of the regions acquired by the measurement deviceis sufficient or insufficient according to predetermined criteria set bythe operator prior to acquisition, whereby said sufficient resolutionprovides enough data for the subsequent inspection task, andinsufficient does not, wherein regions where no data have been acquiredare indicated on the CAD with a marking, acquired regions of sufficientresolution are indicated on the CAD model using a first alternativemarking, and acquired regions of insufficient resolution are marked onthe CAD model using a second alternative marking that is different fromthe first alternative marking, c) re-directing the measurement deviceover the acquired regions indicating insufficient resolution for thesubsequent inspection task, d) updating the indication of the resolutionof the acquired regions on the CAD model, and e) repeating steps c) andd) until sufficient resolution for the inspection task is indicated onthe CAD model, thereby acquiring the dimensions of the object atsufficient resolution for the subsequent inspection task.
 2. Methodaccording to claim 1, wherein the CAD model and indication of resolutionfor acquired regions of the object are displayed using a display means.3. Method according to claim 1, wherein the indication of resolution isupdated after the measurement device acquires a region that at leastpartially overlaps a previously acquired region.
 4. Method according toclaim 1, wherein the measurement device is a manually operatedmeasurement device.
 5. Method according to claim 1, wherein themeasurement device comprises an optical scanner and either a co-ordinatemeasuring machine, CMM, arm or an optical CMM.
 6. A computer programstored on a non-tangible computer readable medium, for the dimensionalacquisition of an object at a resolution sufficient for a subsequentinspection task by a manual dimensional measurement device that isdirected over the object, configured to: a) receive three-dimensionaldata of the object from the measurement device directed over regions ofthe object, b) provide during acquisition, on a nominal computer aideddesign (CAD) model of the object, an indication to the operator ofwhether the resolution of the regions acquired by the measurement deviceis sufficient or insufficient according to predetermined criteria set bythe operator prior to acquisition, whereby said sufficient resolutionprovides enough data for the subsequent inspection task, andinsufficient does not, wherein regions where no data have been acquiredare indicated on the CAD with a marking, acquired regions of sufficientresolution are indicated on the CAD model using a first alternativemarking, and acquired regions of insufficient resolution are marked onthe CAD model using a second alternative marking that is different fromthe first alternative marking, c) receive additional three-dimensionaldata of the object from the measurement device for the acquired regionsindicating insufficient resolution for the subsequent inspection task,d) update the indication of the resolution of the acquired regions onthe CAD model, and e) repeat steps c) and d) until sufficient resolutionfor the subsequent inspection task is indicated on the CAD model. 7.Computer program according to claim 6, comprising a coverage moduleconfigured to receive the three-dimensional data of the object from themeasurement device during dimensional acquisition, and to provide theupdated indication of the resolution of the acquired regions. 8.Computer program according to claim 7, further comprising a feedbackmodule configured to provide said indication of resolution to theacquired regions from the coverage module onto a CAD model.
 9. Computerprogram according to claim 8 wherein acquired regions of sufficientresolution are indicated on the CAD model using a first marking, andacquired regions of insufficient resolution are marked on the CAD modelusing a second marking that is different from the first marking. 10.Computer program according to claim 6, further comprising a displaymeans to display the resulting CAD model.
 11. Computer program accordingto claim 6, further comprising a sub-routine to indicate when thedimensional acquisition of the object to a sufficient resolution for thesubsequent inspection task is complete for all acquired regions of theobject.
 12. Computer program according to claim 6, wherein the feedbackmodule runs synchronously or asynchronously with the coverage module.